A few months ago I parked in the Shoreham branch of Dunelm Mill. I shopped there, buying a picture frame and afterwards left the car in the car park to take my son across the road to play in the nearby park. There were lots of empty spaces so I didn't feel I'd be preventing anyone else from parking.

Although there were signs saying fines would be imposed for parking if not using Dunelm, my brother-in-law had told me this was illegal, although he's not really an authority on this.

On returning to the car I had a parking ticket. My brother-in-law advised me not to pay, but after several letters from the owners of the car park I received a letter from Roxburghe Debt Collectors. The £198 "fine" needs to be paid within 10 days. Should I pay? Are they allowed to do this? SC, Brighton, East Sussex

In recent months we've had a few letters along these lines and thought it would be good to clarify the rules, particularly as stores are increasingly using tactics like this. Although it is never a good idea to ignore such demands, your brother-in-law is correct in that the store, or private company, can't issue parking tickets in the same way as a local authority can.

The UK's leading parking ticket expert, Barry Segal, on his excellent website, appealnow.com, says any penalty they seek to impose is redress for the breach of contract, and as such, only enforceable by a court. The company would have to successfully sue you for breach of contract to get you to pay up. And even in cases in which the parking company has won, courts have been saying that the amounts claimed are not fair, and have reduced them. If these firms threaten to send round debt collectors, as happened to SC, they can't enforce a payment until they win in court, Segal says.

Some try it on, he says, but it is not an enforceable debt until a court declares a judgment against you.

"I would not open the door – tell them they are demanding money with menaces and are trespassing and you will have them arrested," Segal says.

He suggests many parking companies try it on with the idea that most people will simply cave in and pay. The firms rarely go to court as the cost will probably outweigh the return. He advises anyone receiving such a ticket to write back saying, "I wish to have my day in court. Please do not send any more threatening letters as it is a criminal offence."

With that cleared up, we contacted Dunelm Mill to ask about SC's ticket. It said: "Dunelm is always concerned if customers feel that they have been unfairly treated. We have the current parking policy in place due to people regularly misusing the car park, for access to the nearby railway station. The car park is managed by TPS and we have advertised this policy with signs around the car park, warning users of the risk of incurring a fine if they park for longer than 1.5 hours. We would be very happy to speak directly with SC to discuss the charge in more detail."

It has since resolved the matter to SC's satisfaction.

We welcome letters but cannot answer individually. Email us at consumer.champions@guardian.co.uk or write to Bachelor & Brignall, Money, the Guardian, 90 York Way, London N1 9GU. Please include a daytime phone number